This invention relates to relatively short peptides, and more particularly to peptides between about 24 and about 30 residues in length, which are naturally available in minute amounts in the venom of the cone snails or analogous to the naturally available peptides, and which include three cyclizing disulfide linkages.
The publications and other materials used herein to illuminate the background of the invention, and in particular, cases to provide additional details respecting the practice, are incorporated by reference, and for convenience are numerically referenced in the following text and respectively grouped in the appended bibliography.
Mollusks of the genus Conus produce a highly toxic venom which enables them to carry out their unique predatory lifestyle. Prey are immobilized by the venom which is injected by means of a highly specialized venom apparatus, a disposable hollow tooth which functions both in the manner of a harpoon and a hypodermic needle.
Few interactions between organisms are more striking than those between a venomous animal and its envenomated victim. Venom may be used as a primary weapon to capture prey or as a defense mechanism. These venoms disrupt essential organ systems in the envenomated animal, and many of these venoms contain molecules directed to receptors and ion channels of neuromuscular systems.
The predatory cone snails (Conus) have developed a unique biological strategy. Their venom contains relatively small peptides that are targeted to various neuromuscular receptors and may be equivalent in their pharmacological diversity to the alkaloids of plants or secondary metabolites of microorganisms. Many of these peptides are among the smallest nucleic acid-encoded translation products having defined conformations, and as such, they are somewhat unusual. Peptides in this size range normally equilibrate among many conformations. Proteins having a fixed conformation are generally much larger.
The cone snails that produce these toxic peptides, which are generally referred to as conotoxins or conotoxin peptides, are a large genus of venomous gastropods comprising approximately 500 species. All cone snail species are predators that inject venom to capture prey, and the spectrum of animals that the genus as a whole can envenomate is broad. A wide variety of hunting strategies are used; however, every Conus species uses fundamentally the same basic pattern of envenomation.
The major paralytic peptides in these fish-hunting cone venoms were the first to be identified and characterized. In C. geographus venom, three classes of disulfide-rich peptides were found: the .alpha.-conotoxin peptides (which target and block the nicotinic acetylcholine receptors); the .mu.-conotoxin peptides (which target and block the skeletal muscle Na.sup.+ channels); and the .omega.-conotoxin peptides (which target and block the presynaptic neuronal Ca.sup.2+ channels). However, there are multiple homologs in each toxin class; for example, there are at least five different .omega.-conotoxin peptides present in C. geographus venom alone. Considerable variation in sequence is evident, and when different .omega.-conotoxin peptide sequences were first compared, only the cysteine residues that are involved in disulfide bonding and one glycine residue were found to be invariant. Another class of conotoxins found in C. geographus venom is that referred to as conantokins, which cause sleep in young mice and hyperactivity in older mice and are targeted to the NMDA receptor. Each cone venom appears to have its own distinctive group, or signature, of different conotoxin sequences.
Many of these peptides have now become fairly standard research tools in neuroscience. .mu.-Conotoxin peptides, because of their ability to preferentially block muscle but not axonal Na.sup.+ channels, are convenient tools for immobilizing skeletal muscle without affecting axonal or synaptic events. .omega.-Conotoxin peptides have become standard pharmacological reagents for investigating voltage-sensitive Ca.sup.2+ channels and are used to block presynaptic termini and neurotransmitter release.
The most widely used pharmacological agent for inhibiting neurotransmitter release at the present time is a peptide, .omega.-conotoxin peptide GVIA, which was isolated from the venom of the predatory fish-hunting snail, C. geographus (1, 2). This peptide binds with very high affinity and specificity to presynaptic voltage-sensitive Ca.sup.2+ channels (2, 3). In synaptic systems from lower vertebrates, the toxin has proven to be a generally potent agent for inhibiting neurotransmitter release. Thus, if the entry of .sup.45 Ca.sup.2+ is monitored in response to depolarization using synaptosomal preparations from frog or chick brain, the inhibition of Ca.sup.2+ entry by the peptide is essentially complete (10).
In mammalian systems, .omega.-conotoxin peptide GVIA is apparently effective on a much more restricted subset of Ca.sup.2+ channels. Thus, while the peptide causes paralysis in teleost, amphibian and avian systems, the toxin has no measurable effect on most mammalian neuromuscular junctions, even at very high doses (4). In addition, the toxin inhibits only a minor fraction of the total depolarization-induced .sup.45 Ca.sup.2+ influx in most rat brain synaptosome preparations (5). These results indicate that at mammalian synapses, there are critically important voltage-sensitive Ca.sup.2+ channels that under physiological conditions are insensitive to .omega.-conotoxin peptide GVIA. It is therefore necessary to develop other ligands in order to study synaptic transmission in mammalian systems.
Every piscivorous Conus species that has been analyzed so far has at least two divergent .omega.-conotoxin peptide sequences, and homologous toxins from venoms of different Conus species exhibit surprising sequence divergence. From the fish-hunting species C. magus, a highly variable Indo-Pacific species, two .omega.-conotoxin peptides were previously described--MVIIA and MVIIB (6, 7).
.omega.-Conotoxin peptide MVIIA has high affinity for the .omega.-conotoxin peptide GVIA high affinity site, the N-type Ca.sup.2+ channel. It is desired to identify and synthesize .omega.-conotoxin peptides which do not primarily target to N-channels in mammalian systems.